1. Field of the Invention
The present disclosure generally relates to microbial control using peracetate oxidant solutions. The disclosure more particularly relates to a method of reducing microbial load, disinfecting, and sanitizing contaminated water involving the use of peracetate oxidant solutions.
2. Description of the Relevant Art
Microbial control in water is imperative to a wide variety of processing and manufacturing systems. These systems can include water recycling loops, pulp and paper mills, cooling towers and water loops, feedstock processing systems, evaporation ponds and non-potable water systems. Treatment of water for microbial control in water recycle loops is critical for maintaining efficient processes, protecting equipment from biofouling and biocorrosion, preventing contamination of products, reducing downtime and protecting the health of people exposed to such processes and products. Furthermore, microbial control in water recycle loops also provides odor control by minimizing fermentation, hydrogen sulfide production and algal decomposition.
Microbial control in pulp and paper mills serves to protect the integrity of pulp slurries, coating ingredients, whitewater loop, process equipment, and paper quality. Controlling sessile bacteria helps to prevent the accumulation of biofilm deposits which cause microbiologically influenced corrosion (i.e., biocorrosion). Slime deposits are often a combination of bacteria and fungi. Importantly, when biofilms and their detritus detach from surfaces in the wet end papermaking process, they can cause holes and other defects in finished paper products. Therefore, preventing biofilm growth helps to avoid such defects.
Microbial control in cooling towers and cooling water loops serves to improve cooling efficiency, minimize microbiologically influenced corrosion, control odors, prevent clogging of pumps and pipes, reduce microbial loading in blowdown, and minimize microbial exposure of surrounding areas from drift.
Microbial control may also occur on surfaces serving to bleach, sanitize and/or disinfect the surfaces of a processing or manufacturing system.
Microbial control targets include aerobic and anaerobic bacteria (slime formers, acid producers, metal depositors, nitrobacteria, sulfate reducers, nitrate reducers), fungi, algae, molds, spores and yeast. Some bacteria are pathogenic, for example, Legionella pneumophila, which poses health risks. Some algae, such as cyanobacteria, produce algal toxins that pose potential health hazards.
Compounds used for microbial control need to be effective and efficient at neutral and alkaline pH. They also need to be effective at elevated levels of suspended solids (including silt, pulp, fillers, pigments, suspended metals, oils, polymers) and dissolved solids (including salt, scaling minerals, carbonate, dissolved metals, scale inhibitors and other additives that may be encountered in various processes).
Microbial control is generally achieved using chemical biocides. Oxidizing biocides (e.g., chlorine gas, chlorine bleach, iodine, hypobromous acid, chlorine dioxide, chloramines, bromamines, fluorine, peroxyacetic acid, hydrogen peroxide, ozone) are typically fast acting and relatively short lived compared to non-oxidizing biocides (e.g., glutaraldehyde, dodecylguanidine, bromohydroxyacetophenone, bronopol, hydantoins, isothiazolins), which are slower acting, but leave long lasting active residuals that can persist for several weeks in the environment. Commonly used oxidizing biocides are effective in the treatment of water with relatively low levels of contaminants, however significant issues arise when higher concentrations of organic materials and salinity are present. Microbial resistance to chlorine and bromine-based oxidizing biocides is a growing issue in municipal and industrial water systems.
There are numerous tradeoffs in selecting a biocide for specific applications. Chlorine was first used in municipal water treatment in the U.S. in 1909 as a disinfectant. Since then chlorine and chlorine-based biocides have been the standard for large scale municipal and industrial disinfection. Oxidizing biocides based on free chlorine and bromine in water react readily with organic materials to form halogenated disinfection byproducts, which are persistent in the environment and often exhibiting high toxicity. The antimicrobial activity of aqueous chlorine and bromine decreases rapidly above about pH 7 and pH 8, respectively. Chlorine dioxide is an effective biocide over a wider pH range and has a lower potential to form halogenated disinfection byproducts if generated properly. However, byproducts of chlorine dioxide include chlorite and chlorate, which are regulated in drinking water. Peroxyacetic acid (PAA), which is a stabilized mixture of PAA, hydrogen peroxide, acetic acid and water, is an effective biocide, but not as efficient as chlorine dioxide in that higher doses are necessary to achieve similar performance. PAA performance declines as pH becomes more alkaline and promotes non-beneficial decomposition reactions between PAA, hydrogen peroxide and metal contaminants. Hydrogen peroxide by itself has significantly lower antimicrobial efficacy than PAA and halogen-based biocides while microbes can rapidly develop tolerance to it in water recycle loops. PAA and hydrogen peroxide rapidly degrade in the environment and form significantly fewer disinfection byproducts than halogenated biocides. Oxidizing biocides can also directly oxidize odor-causing materials such as phenols, sulfides and mercaptans.
Corrosivity of oxidizing biocides is another issue, especially when the biocides come in contact with various process materials such as steel, copper and brass alloys. Oxidizing biocides used in processes where elevated temperatures and turbulence are present in the liquid phase should ideally have low vapor pressures to minimize vapor phase corrosion of surrounding equipment and structures. Biocide materials that are gases in their native form are the most volatile and present the greatest corrosion and occupational exposure hazards, including chlorine, chlorine dioxide and ozone.
Control of biocide dosing in a process stream by monitoring the oxidation potential of the treated water is an advantage for real-time process control. The oxidation-reduction potential (ORP) of a solution can be correlated with a level of biocidal control at a given pH and often with the concentration of active biocide present (and corresponding corrosivity). Various forms of chlorine, bromine, chlorine dioxide and sometimes ozone can provide a strong ORP response when used at low concentrations at neutral to moderately alkaline pH. For example, the ORP of chlorine bleach or chlorine dioxide at a 1-2 ppm concentration in relatively clean fresh water at pH 7 can exceed 700 mV vs standard hydrogen electrode (ORP greater than 650 mV typically provides effective bacteria control). In contrast, PAA, hydrogen peroxide and non-oxidizing biocides do not provide a meaningful ORP response above a dissolved oxygen background in fresh water, which is about 420-520 mV at pH 7.
There is a need for highly effective and fast acting oxidizing biocides that are safer to use, have lower environmental impacts and contribute to pollution prevention efforts. Water-based alkyl peroxide salt solutions that efficiently produce reactive oxygen species (ROS) are a class of highly active oxidants that provide multiple biocidal species, have low volatility, degrade to benign residuals, can be produced from stable feedstocks under mild conditions, and reduce or eliminate several harmful disinfection and oxidation byproducts.
It is desirable to find an efficient and cost effective method of microbial control in water of process systems.